(A) Field of the Invention
The present invention, which relates generally to liquid pumping units comprising multiple centrifugal pump elements incorporated in a common body, is particularly concerned with apparatus for controlling the passage of the liquid between several channels inside the pump body and with the adjustment of the flow-rate produced by these pumping units. The present invention is also concerned with the mounting of these passage and flow-rate control devices in a pumping unit, such as a central-heating booster pump.
(B) Discussion of the Prior Art
In order to reduce the cost of industrial or domestic heating installations, it is advantageous to fabricate pumping units which comprise several discrete pump elements combined in a common pump body. Advantageously, the pump elements are mass-produced and they need not have the same shape and operating characteristics. Manufacturers currently produce modules which group two, three or even more similar pump elements in modular form. The grouping of two identical pump elements in a common pump body is perhaps the most common configuration for such pumping units which are known as twin pumps. When used for industrial or domestic central heating, they are commonly known as twin booster pumps.
The twin pump or booster pump is advantageous since it assures high reliability for the heating system in which it is installed. Consider, however, an installation using a single pump which has failed. Replacing the failed pump by a new pump is in itself an easy operation, but requires that the heat be turned off and the pipes drained. These factors seriously disturb operation, especially during cold spells. The use of a twin pump, however, protects the installation from such problems. The centrifugal pump elements are typically identical. Their operating characteristics, and in particular the power of each, are selected so as to satisfy the requirements of the installation. The twin pump is intended for alternate operation, i.e. each pump element is operated alternately. In the event of a failure in one of the elements, the other automatically takes over.
In general, in order to avoid unequal wear of the pump elements, both elements are operated on a 50% duty cycle. The work loads are thus balanced. Naturally, if one element fails, the installation is not disturbed in any way. Normal operation can quickly be re-established by merely replacing the defective pump element with a new element, without draining the plumbing. Finally, the use of a twin pump considerably reduces the risk of failure, since it is extremely rare for both elements to fail simultaneously. This conventional solution is today used frequently in most new installations.
For the manufacturer, the production of these pumping units raises two kinds of problem. First, it is necessary to provide a pump element which alternates the flow in the channels feeding the pumps in a virtually automatic manner. Second, it is advantageous to provide a pump component to vary the flow-rate of either pump in order to adjust the flow-rate to the installation served. The flow-rate is controlled by an adjustable shutter, known as a variator, located in a duct known as the discharge duct between the outlet and inlet. At first blush, this conventional arrangement would seem to call for as many variators, on as many discharge ducts, as there are pump elements.
In order to satisfy the first of the above requirements, the manufacturer typically incorporates valve systems in the common pump body to automatically control the alternated passage of liquid to or from either element.
The French Pat. No. 2,105,733 filed by the present applicant, discloses an adjustable-flow, multiple-booster pump, including a twin booster in which the element which controls the passage of the liquid in the outlet is a spring-loaded valve 22. As shown in FIG. 4 of that patent which is hereby incorporated by reference, the valve automatically controls the outlet 23' in volute 18' of the left pump element when the right volute 18 delivers liquid to the common outlet chamber to the general orifice 15. This valve, which is freely mounted on a shaft perpendicular to the plane of the figure, is held in position by a return spring designed to yield to the liquid pressure on the surface opposite that on which the return spring is mounted. On the inlet, the supply is controlled by a four-way valve, as shown in FIG. 1.
In order to meet the second of the above requirements, the above-mentioned patent describes, for regulating the twin booster output, a single variator whose control knob 9 operates simultaneously on the position of two planetary parts, each installed in a manner providing communication between the inlet duct 20 and outlet duct 21 for the corresponding pump element. The variator described simultaneously provides effective regulation of the flow of both pump elements by means of a single control and, therefore, regulation of the twin booster pump output.
The saving in space which is achieved as a result of replacing at least two complete pumps connected in cascade by a pumping unit containing at least two pump elements incorporated in a common body is nervertheless still limited, since it is necessary to adapt to the common body on the outlet side, according to the invention mentioned above, at least one system of valves and at least a control mechanism with complex gears on the discharge ducts of the pump elements.